This invention relates to an improved process for continuously providing plastic films with protective coatings. More particularly, it relates to a method for using a silicone resin coating composition under conditions which make the use of conventional film converting machinery practicable.
Silicone resins, especially silicone hard coating resins, are notoriously slow curing materials. The latter comprise aqueous compositions based on a dispersion of colloidal silica in an aliphatic alcohol-water solution of the partial condensate of a silanol. These find use as mar and scratch resistant coatings for plastic substrates in the form of sheets and plaques, such as polycarbonate and acrylic windows in transportation equipment, and in plastic lenses, such as acrylic eye-glass lenses, and the like. Such compositions and their use are described, for example, in Clark, U.S. Pat. Nos. 3,986,997; in Ubersax, 4,177,315; and in the co-pending U.S. patent applications of James T. Conroy, Ser. No. 170,994, filed July 18, 1980, now U.S. Pat. No. 4,311,763, patented Jan. 19, 1982, and Ser. No. 172,269, filed on July 25, 1980, the disclosures of which are incorporated herein by reference.
The above-mentioned slow curing of such compositions is detrimental to their use where high line speeds are required, such as in film-coating operations. By way of illustration, uncatalyzed polysilicic acid-silanol compositions require about two minutes to reach full cure in a 150.degree. C. forced air circulating oven. Full cure is critical because, without it, the coating will not have good adhesion to the substrate and good mar resistance.
Film converters (fabricators) process plastic films on web coating machines that are operated at line speeds of 100 feet per minute and higher. Typically, these machines have ovens 50 feet in length for use in solvent removal and/or curing of the coatings. Under these conditions, the maximum allowable processing time for a coating is 120 seconds, and usually even less, 30 seconds or so.
There has now been discovered a way to greatly accelerate the cure of such silicone hard coatings so that they can be processed at high line speeds on films such as poly(ethylene terephthalate). If, for example, an alkali alkanoate salt or a quaternary ammonium salt, e.g., acetate, are selected and added in optimum amounts as catalysts to the coating compositions, and .beta.-hydroxyketo compounds are added to promote flow, formulations are obtained which can be cured in 25-30 seconds at 150.degree. C. in a forced air circulating oven. Although the references cited above, in general, disclose catalytic activity with salts of the type discovered to be active herein, and the promotion of adhesion with .beta.-hydroxyketone compounds, it is believed that it is unexpected to find that the use of catalysts in combination with flow promoters makes possible the use of such compositions in the film converting industry with its unique requirements.